How do I know what they found or viewed?
If they did it the forensically correct way, then there is NO WAY to determine what was examined. The correct way would have been for the forensic tech to pull the hard drive out, clone it, return your hard drive to the machine and then examine the clone image. There will be no traces whatsoever on your drive since it was never booted. They will work strictly from the clone image and you should consider everything on the hard drive as having been compromised and you also should be aware they may have retained the image or parts of the image. Hope you didn't have anything on there you will regret.
The quickest way to view the time your files were last accessed, as far as I know:
Download the search tool Everything: http://www.voidtools.com/
Install it and run the program, and wait for it to index your disk (should take less than a minute)
Richt-click on the column headers (where it says "date modified"), and enable "Last accessed"
Now search for something random, like
Click on the new column header "Last accessed" to sort search results, so that latest are on top
Delete your old search, and search for
*.*. This will take a long time, especially if this is the first time you use Everything; it could be up to 10 minutes, I don't know how long: that depends on your PC; just keep waiting, it took me 3 minutes on this old-ish PC
Now you have a list of all the files on your PC, listed by the time they were last accessed, with dates and times.
Note that Windows also accesses some files when the PC is on without external output, so it you can't be sure it was they who accessed a file.